1. Field of the Invention
The present invention relates to a method of repairing a defect in a photomask. More particularly, the present invention relates to a method of repairing an opaque defect in the region of a photomask defining an opening through which exposure light is transmitted.
2. Description of the Related Art
The fabricating of an integrated circuit generally includes a plurality of photolithography processes that form circuit elements on the surface of a semiconductor substrate. A high quality photomask is a prerequisite for highly reliable photolithography. In particular, a photomask must have no defects and must be able to define a fine pattern on a semiconductor substrate in order to form a highly-integrated circuit. Typical defects in photomasks can be generally classified as either clear or opaque defects. A clear defect refers to damage in the portion of the photomask that defines the pattern that is to be reproduced on the substrate. On the other hand, as shown in FIG. 1, an opaque defect 30 refers to the presence of a film of light shielding material or phase shifting material on a light transmission portion 25 defining an opening through which light should pass. The opening is defined by a light shielding material pattern or a phase-shifting material pattern 20 formed on a transparent substrate 10.
The prevailing method for repairing the opaque defect uses a focused ion beam (FIB). In this method, the opaque defect 30 is etched and removed by projecting an FIB 40 onto the opaque defect 30 only, as shown in FIG. 2.
However, as shown in FIG. 3, when the opaque defect 30 is removed by the FIB, a recess (R) is formed in the substrate 10 along the edge of the opaque defect. The recess (R) is created because the region of the substrate 10 at the edge of the opaque defect 30 is also exposed tot eh FIB while the opaque defect is being etched. The recess (R) becomes even worse when the sidewall of the opaque defect 30 is not completely perpendicular but is inclined as shown in FIG. 2. Also, the recess (R) becomes deeper when an etching process is performed to remove an ion stain formed when the ions used to etch the opaque defect remain on the surface of the substrate 10 in the light transmission portion 25. Accordingly, a large step difference (d) is created in the light transmission portion 25 between a normal substrate region and the region of the substrate where the defect existed.
Therefore, when photolithography is performed using a photomask repaired by the conventional method, light is scattered by the recess (R), and a phase shift is produced due to the step difference (d). This reduces the transmissivity of the light transmission portion 25. This phenomenon becomes more serious in the case of a photomask that must have a constant light transmissivity characteristic and a small critical dimension (CD). Therefore, in many cases, it is almost impossible to use photomask repaired by the conventional method.
An object of the present invention, therefore, is to provide a method of repairing a defect in a photomask, which minimizes the forming in the light transmission portion of a step between a normal area of the substrate and the area where the defect was formed, and which prevents a recess from being formed in the substrate within the light transmission portion.
To achieve this object, the method of the present invention includes a pre-etching step, a correction film formation step, and a final etching step. In the pre-etching step, the thickness of the opaque defect is reduced by etching away only some of the opaque defect. In the correction film formation step, a correction film is selectively formed over the entire surface of the substrate within the light transmission portion with the exception of the region on which the pre-etched defect resides. In the final etching step, both the correction film and the pre-etched opaque defect are completely removed by performing an etching process over only the light transmission portion.
Preferably, the entire region of the light transmission portion is etched in the pre-etching step. Also, the light transmission portion is rectangular, and the final etching step is preferably performed by scanning the light transmission portion over a rectangular area corresponding to the shape of the light transmission portion. In this way, the corners of the light transmission portion are squared off, if necessary, to maximize the transmissivity of the photomask.